This invention relates to a driving method of an electrooptic device including pixel electrodes and nonlinear resistance elements arranged as to define pixels along driving electrodes.
Among the various display panels known in the art, the liquid crystal type display panel is excellent because it can be thin in thickness, light in weight, and low in power consumption. Thus, the liquid crystal display panel is widely used in computers of the lap-top type, note book type, and the like. In particular, the active matrix type display panel is attractive because it is capable of handling a large volume of display information and attaining a high degree of picture quality. The active matrix display panel use the three-terminal type active element which is a thin-film transistor, and the two-terminal type active element which is a nonlinear resistance element such as MIM or a PN junction thin-film diode.
The three-terminal element needs a number of film formed. Thus, its manufacturing process is complicated, yield is low, and cost is high. The diode has a characteristic of low operation voltage and weak resistance to static electricity. Contrarily, the nonlinear resistance element is simple in structure and can be operated at high voltage, thus can be advantageously used in large-size display panels without increasing cost.
The conventional electrooptical device using nonlinear resistance elements has a structure such that an electrooptical liquid crystal material is sealed between two opposing substrates on which column and row electrodes are formed respectively, and nonlinear resistance elements and pixel electrodes are formed on the inner surface of one of the substrates. The nonlinear resistance element is connected between the pixel electrode and the row or column electrode. This type of electrooptical device is disclosed in U.S. Pat. No. 4,871,234.
In order to display on this liquid crystal panel, it is important to determine a driving voltage and, the correct composition and thickness of the nonlinear resistance layer so as to obtain desired resistances of nonlinear resistance element during driving with an applied voltage. It is also important to increase a ratio of a capacitance of a liquid crystal portion of each unit pixel to a capacitance of a nonlinear resistance element portion so as to obtain a sufficient operating margin and to compensate for a distribution of element characteristics and deviations over time. In recent years, as the display panel using nonlinear resistance elements has become large, a problem develops during gray scale display.
In the nonlinear resistance element, however, a very small current (up to about 10 pA) flows even during the retention period. Stored data is affected gradually by data signals applied to a corresponding column electrode. For this reason, the RMS voltage applied to the liquid crystal in accordance with a display pattern gradually deviates from the predetermined value. In addition, since the resistance of the element greatly influences a charge injection capacity and a charge retention capacity, element characteristics vary within the panel surface and are shifted due to the deteriorations over time. At this time, changes in element characteristics cause a direct change in the RMS voltage applied to the liquid crystal. For this reason, when an RMS voltage applied to the liquid crystal is to be controlled with high precision as in a multilevel gray scale display, a contrast difference is caused to make it difficult to perform a normal display. This difference is increased when the panel size is increased and the number of dots is increased, resulting in difficulty and inconvenience.